Research into the utilization of corneal grafts and implants to affect corneal curvature began with the work of Dr. Jose Barraquer of Bogota, Columbia and others prior to 1950. Dr. Barraquer's research involved the use of implanted donor corneal lenticules that had been removed from a donor eye with a microkeratome, frozen, lathed and placed within the recipient cornea to achieve a steepening of the anterior corneal curvature. Within the last two decades, research has been done in determining compatible implant materials which may be successful as synthetic corneal implants for the correction of hyperopia. A number of researchers have determined that a highly compatible and desirable implant may be made from a hydrogel material. See inter alia, Werblin, et al., "Stability of Hydrogel Intracorneal Implants in Non-Human Primates," CLAO Journal 9:157-161 (1983). Binder, et al., "Hydrogel Keratophakia in Non-human Primates," Current Eye Research, 1:535-42 (1981); Koenig, et al., "Refractive Keratoplasty With Hydrogel Implants in Primates," Ophthalmic Surgery, 15:225-229 (1984). It has been recognized that the objective of such hydrogel implants is to alter the anterior corneal curvature to hopefully provide a predictable, stable and reversible change in refraction. Because of the large difference in the refractive indices between the precorneal tear film and air, small changes in the corneal radius will result in relatively large changes in refractive power. Hydrogel materials have been found to be sufficiently permeable to water and metabolic nutrients to maintain normal corneal physiology. For the most part to date, however, that work has been carried out in order to provide steepening for hyperopic correction.
It has been determined that the more desirable surgical technique for the implantation of a hydrogel lens is the surgical procedure which utilizes the Barraquer microkeratome for lamellar keratectomy. Beekhuis, McCarey, et al. "Hydrogel Keratophakia: A Microkeratome Dissection in the Monkey Model," British Journal of Ophthalmology, 70:192-198 (1986); McCarey, et al., "Hydrogel Keratophakia: A Freehand Pocket Dissection in the Monkey Model," British Journal of Ophthalmology," 70:187-191 (1986); Sendele, et al. "Intracorneal Lens Implantation," Arch Ophthalmology, 101:940-944 (1983). It has been suggested that the main reason for use of microkeratome as compared to a pocket dissection is the necessity of severing the anterior collagen bundles and Bowman's layer in order to allow these structures to assume the anterior corneal curvature change imposed as a result of the implant. Binder, et al, "Hydrophilic Lenses for Refractive Kerotoplasty: The Use of Factory Lathed Materials," CLAO Journal 10:105-111 (1984); McCarey, et al., "Hydrogel Keratophakia: A Freehand Pocket Dissection in the Monkey Model," British Journal of Ophthalmology," 70:187-191 (1986).
Over these three and one-half decades of research, virtually all of the work has related to correction of hyperopia. In the correction of hyperopia, the purpose of the implant is to steepen the corneal curvature. There has been much less research activity with the even more complex problem of utilizing corneal implants to correct myopia. Werblin, et al., "Myopic and Hyperopic Hydrogel Keratophakia," Arch. Soc. Oftal. Optom., 18:131-143 (1984); Werblin, et al., "Myopic Hydrogel Keratophakia: Preliminary Report," Cornea, 3:197-204 (1985); Werblin, et al., "Myopic Correction Using Alloplastic Implants in Non-Human Primates--A Preliminary Report," Annals of Ophthalmology, 1127-1130 (1984). This is in part because the corneal curvature must not be steepened as in the correction of hyperopia but rather the corneal curvature must be flattened for the correction of myopia. It has been concluded that negative refractive lenses can be implanted into the cornea of primates in order to produce a substantial corneal flattening. While the initial flattening produced by research was irregular, it has been suggested that enlarging the size of the lenticule and the optical zone could possibly produce a corneal flattening which would become more regular. For primates, positive results were obtained utilizing lenses having a diameter of 6.75 mm as compared to prior results utilizing implants at 5.5 mm. Werblin, Patel, "Hydrogel Keratophakia: Improvements in Myopic Lens Design," ARVO Abstracts 1986:14.
While prior research has pointed the way toward the most desirable surgical procedure using the microkeratome and at least one school of researchers has concluded that hydrogel materials are prime candidates to be utilized as corneal implants, little work has been done to actually design a corneal implant lens having negative refractive power in order to correct myopia in humans. Some work has been done in an effort to predict the ultimate corneal refractive alteration caused by hydrogel keratophakia. In the paper entitled "Predicting Refractive Alterations With Hydrogel Keratophakia," authors Watsky, McCarey and Beekhuis presented algorithms that utilized elementary optics to Calculate the total corneal power produced through intracorneal lens implantation via either pocket or microkeratome dissection. Investigative Ophthalmology and Visual Science, 26:240-243 (1985). In the paper "Alloplastic Refractive Keratophalia: A Comparison of Predictive Algorithms," the authors Watsky and McCarey compared the accuracy of three different algorithms to predict actual in vivo power of a lens: (1) a back vertex power algorithm ("sandwich" algorithm); (2) paraxial ray tracing and (3) meridional ray tracing for implanted positive hydrogel and polysulfone implants. See also, Churms, "The Theory and Computation of Optical Modifications to the Cornea and Refractive Keratoplasty," American Journal of Optometry and Physiological Optics, 56(2):67 (1979). See also, Mester, et al., "Measurement and Calculation of Refraction in Experimental Keratophakia With Hydrophilic Lenses," Opthal. Res. 8:111-116 (1976); Arffa, et al., "Keratometry in Epikeratophakia," Journal of Refractive Surgery 2:61-64 (1986).
U.S. Pat. No. 4,607,617 of Choyce entitled "Apparatus and Method of Improving Eyesight," discloses a corneal implant made of a polysulfone plastics material designed to be implanted between layers of the cornea to correct eyesight defects. The nature of the polysulfone material disclosed in the '617 patent is quite different from the hydrogel materials previously discussed in that the polysulfone material has a high refractive index relative to that of the cornea. Further, it is believed that the hydrophilic, water permeable nature of the hydrogel allows more flexibility in placement of an implant as compared to the implanting of polysulfone that is not water impermeable. U.S. Pat. No. 4,612,012 of White is directed to a prosthesis which supposedly provides for the surgical replacement of all or a portion of the cornea, the implant having a peripheral portion having an outer tissue contacting surface of a biologically compatible material. U.S. Pat. No. 4,298,004 of Schachar, et al., discloses a surgical material wherein collagen or other suitable material is injected into an incision in the cornea in order to alter the radius of curvature.